D O C U M E N T 6 5 F E B R U A R Y 1 9 2 1 5 5 of course, causally determined only insofar as it exhibits characteristics indepen- dent of the choice of a coordinate system. Questions such as where does the grav- itational field that forms when I set the coordinate system into rotation come from? are therefore senseless. Experiments like the one you suggest are, in principle,[3] thoroughly legitimate. The gen. theory of relativity allows an exact calculation of the result in advance. Unfortunately, all the effects coming into consideration here enter into the factor: gravitational constant/square of the velocity of light, which is of order of magni- tude 10–28 and causes these effects not to be attainable by laboratory experiment because of their smallness.[4] In gr. respect. 65. From Paul Oppenheim Frankfurt am M[ain], 46 Guiollett Street, 25 February 1921 My dear Professor, I had just written the enclosed protest letter to Mr. Fabre[1] when his book arrived, which I send you herewith. My letter is thus, unfortunately, outdated and I ask you now please kindly to let me know what I ought to write with regard to pages 15–18.[2] I find the author’s whole proceedings obnoxious and his enthusiasm is absolutely no excuse; I rather see in this an unacceptable exploitation of your name for commercial purposes, as it has unfortunately probably not been the first time. I am, naturally, innocent of the whole indiscretion, otherwise I would be even more annoyed about it.[3] I would, of course, not like to answer Mr. Fabre’s letter, which I received before my trip to Berlin, before I have your response about what to write him, and would therefore be obliged to you if you let me know as soon as possible what and how I ought to reply. I would naturally very much prefer it if you wrote him directly; for one can see from this case what can happen when someone stands in-between, even when he, like I, is so careful and devoted to you; one formal rea- son you can have for writing him directly is the fact that I forwarded a copy of his letter at his request. If, however, you for any reason prefer not to correspond with such a person directly anymore, please feel free to make use of my services as inter- mediary. One could also consider sending out my letter as if I had not yet received his book; I would not find this altogether unprincipled, as I had demonstrably writ- ten him before receipt of the work. If you think this approach practicable, I request that you return my letter addressed to Mr. Fabre: otherwise, please destroy it. If you would like to read the book itself before returning it to me, there is, of course, plenty of time for that.